A typical airlift pump, illustrated in FIG. 1, consists of an open ended vertical riser tube, partially submerged in a liquid and near the bottom of which compressed air or another gas is injected. Once the air or other gas is injected into the liquid, the average density of the air-liquid mixture in the riser tube becomes less than the density of the surrounding liquid. The resulting buoyant force causes a pumping action in the riser tube and the air-liquid mixture is discharged at the top of the riser tube.
Airlift pumps are widely applied for the purpose of pumping liquids. For example, an airlift pump is described in the article entitled "Airlift Pump Cleans Around Subsea Wellhead", Oil and Gas Journal, Volume 90, Aug. 3, 1992, pages 62-64. A lift pump operation is also explained in reference to FIGS. 45 and 46 in Encyclopedia Of Fluid Mechanics, Volume 2, Dynamics of Single Fluid Flows and Mixing, Nicholas P. Cheremisinoff, Editor.
In the article entitled "Simulation Of Airlift Pumps For Deep Water Wells", The Canadian Journal Of Chemical Engineering, Volume 74, August, 1996, pages 448-456, mention is made that airlift pumps offer simplicity of construction and lack of moving mechanical parts as their main advantages. This article also illustrates both external airline and internal airlift pump systems. In another article entitled "Explore The Potential Of Airlift Pumps And Multiphase", Chemical Engineering Progress, August 1993, pages 51-56, a discussion is given of the effect of liquid density on the operation of an airlift pump. A discussion of how the ratio of the length of submerged riser tube to the total riser tube length affects the airlift pump performance is found in the article entitled "Visual Study Of An Airlift Pump Operating At Low Submergence Ratios", The Canadian Journal Of Chemical Engineering, Volume 73, October, 1995. In a wastewater system marketed as the Nibbler, Jr..TM. by Northwest Cascade-Stuth of 16207 Meridian, Puyallup, Wash. 98373, an airlift system is used to lift wastewater.
In the particular context of treatment of wastewater, it is recognized by the present invention that the typical airlift pump tends to aerate the liquid being pumped and that aeration of the wastewater could become an advantage when the wastewater is liquid being pumped from a septic tank to a drain field having a clay-like character which makes liquid that is not aerated difficult to absorb. Also recognized by the invention is the potential advantage of pumping wastewater to a drain field intermittently rather than constantly so as to permit the drain field to absorb intermittently and the drain field pipes to be flushed intermittently.
In the typical application of an airlift pump, the inflow to the tank, vessel or other body holding the liquid to be pumped and in which the riser tube is submerged, the air flow rate and the depth of submergence of the riser tube are all relatively constant and the airlift pump outflow is continous. So far as is known, an airlift pump has never been constructed such that without requiring moving parts or controls, it can be made to pump intermittently. While the conventional airlift pump construction and operation is well understood, what has not been recognized is that by making a significant modification of the conventional airlift pump construction as provided by the present invention, the airlift pump can be made to operate intermittently. It has been shown that the airlift pump of the present invention, under selected conditions, is significantly more efficient than traditional airlift pumps, providing for increased output flow rate.
The airlift pump of the present invention may be particularly suited for many applications, including subsea wellheads, water wells, and like applications. The particular application described in detail herein is for pumping and aerating household wastewater in septic tank systems, it being understood that the airlift pump so described is equally applicable to many different applications.
With the foregoing in mind, the object of the present invention becomes that of providing an improved airlift pump apparatus and method for general application.
A further object of the present invention is to provide an airlift pump apparatus that operates using reduced air supply power.
Another object of the present invention is to provide an airlift pump apparatus and method, wherein the discharged liquid is caused to be emitted in intermittent powerful bursts.
Yet another object of the present invention is to provide an airlift pump apparatus and method, wherein the rate of liquid output is increased compared to the rate of liquid output of a typical airlift pump.
Yet another object of the present invention is to provide an airlift pump apparatus and method that is suited to the needs of pumping wastewater from septic tanks to drain fields, wherein the liquid is pumped intermittently, but substantially evenly over the course of a day during which liquid input to the septic tank may vary substantially and is also pumped in a manner which causes the discharge water directed to the drain field to be highly and intermittently aerated and therefore more suitable for absorption by clay and other soils of like character.
Other objects will become apparent as the description proceeds.